Add a member function to load image data from pixel unpack buffer

[scheibel]

This would include a duplication of all imageData (and probably imageStorage) functions. As a workaround, use this template instead: /* assume texture and pbo to a validly configured Texture and Buffer */ pbo->bind(GL_PIXEL_UNPACK_BUFFER); texture->[imageFunction]([parameters], nullptr); pbo->unbind(GL_PIXEL_UNPACK_BUFFER);